Method of heating magnesium alloy billets



April 2, 1957 K. F. BRAEUNINGER ETAL 2,

METHOD OF HEATING MAGNESIUM ALLOY BILLETS Filed Oct. 20, 1954 E U T A R E P M v E T m M T U N E A C R w M m, E m W w IJ O O O O O 0 O O 0 0 O O 0 0 O O 0 0 mo wmzbhmmmimk TIME-MINUTES ATTORNEYS METHOD OF HEATING MAGNESIUM ALLOY BILLETS Karl F. Braeuninger and Clifton J. Huffman, Ferguson,

Mo., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application October 20, 1954, Serial No. 463,574

4 Claims. (Cl. 21910.41)

This invention relates to a method of heating thick billets of magnesium alloys, and particularly to a method of inductively heating magnesium alloy billets which are more than 16 inches thick.

Billets of magnesium alloy are usually raised to an elevated working temperature before being rolled, extruded, or otherwise worked to shape or heat treat the alloy. The heating of the billet may be done in an oven, by direct gas flame heating, by induction heating, or by other suitable means. Whatever the means utilized to preheat the billets, the preheating operation represents a considerable expense in view of the heating equipment and the power required to do the actual heating.

Many magnesium alloys must be raised to working temperatures which are in or above the temperature range where hot-short cracking of the billet occurs. Consequently, care must be taken in order to prevent radial cracking of the billet while it is heated through its hotshort temperature range where the cohesion between crystals in the alloy is decreased.

Magnesium alloys of the magnesium-aluminum-zinc ternary variety containing about 3 to percent of aluminum exhibit a tendency to hot-shortness at about 650 F, probably due to a molten phase in the alloy at or near to that temperature. Thus, an alloy in its hot-short temperature range has little resistance to internal tension which builds up as the temperature of the central part of the billet rises.

In the past, radial cracking of magnesium alloy billets which are heated through their hot-short range of temperatures has been controlled either by heating the billet slowly or by rapidly heating the billet in steps which are followed by temperature equalization periods. The center-to-surface temperature differential of the billet is no more than about 100-120 F. when either method of billet heating is used. These methods of billet heating prove generally satisfactory for billets up to about 16 inches thick.

For billets of greater thickness, however, the billet heating time becomes excessive if a low center-to-surface temperature difference is maintained, especially when the heating time is compared with the rapidity with which the heated billets are worked after heating. The result is, if billet heating methods of the past are used, that an increased number of billet heating units must be provided in order to supply suflicient billets to keep the alloy working equipment in continuous operation. For example, an extrusion press using 26 inch diameter billets would, in view of the long time required to heat a billet to its Working temperature, require considerably more billet heating units than would be required for the operation of a press using, say, 12 inch diameter billets.

It can be appreciated that increasing the number of billet heating units is expensive both in itself and in the additional floor space required for the units.

Accordingly, an object of this invention is an improved method of rapidly heating billets of magnesium alloys 'ice to working temperatures which are in or above the hotshort range of the alloy.

In accordance with the method of this invention, the surface temperature of the magnesium alloy billets to be heated is elevated rapidly to near the hot-short temperature range of the alloy and then the surface temperature is raised at a rate which is a small fraction of the initial heating rate until the surface-to-center temperature difference of the billet is less than about 120 F.

The present invention will be described in connection with induction heating of billets, although the invention is also applicable to direct flame gas heating of billets in which a multiplicity of gas flames impinge on the outer surface of the billet.

The inductive heating of a billet is accomplished by surrounding the billet with a coil through which alternating electric current is passed. Induced current flows in a circumferential direction around the billet, with the density of the induced current being greatest at the surface. The depth of the penetration of the induced current depends on the frequency of the alternating current in the induction heating coil which surrounds the billet, with the depth of penetration increasing as the alternating current in the coil decreases in frequency.

in the case of thick billets, the heating of the center part of the billet depends on thermal conduction from the surface part of the billet. In the practice of this invention, and as indicated in the drawing which illustrates the invention as applied to a 26 inch diameter billet of A261 alloy, the temperature differential between the surface and the center of a billet or ingot may be 500600 Fm, depending on the rate of power input to the coil, the rate of thermal conduction through the billet, and the diameter or thickness of the billet. The high surface-to-center temperature differential results in a faster heat transfer to the center of the billet.

In accordance with the invention, the surface temperature of the billet to be heated is raised, by induction heating, for example, at a rate of approximately F. per minute until the surface temperature is near to the hot-short temperature of the billet. For magnesium alloys containing from 3 to 10 percent aluminum, the hotshort temperature (at the lower end of the hot-short range) is about 650 F. The heating rate is then reduced, and the surface temperature is raised at a rate of about 5 to 10 F. per minute until a suitable temperature differential exists between the surface and center of the billet, that is, until the surface-to-center differential is less than F. for magnesium alloys containing from 3 to 10 percent of aluminum.

It is important in this invention that the surface temperature be rising while the billet is in the hot-short temperature range until the 120 F. or less temperature differential referred to above is maintained. It is thought that raising the surface temperature while the billet is in the hot-short temperature range allows the surface to be under compression at all times, and thus strongly reduces the tendency for radial cracking due to tensions resulting from the heating of the center of the billet.

The method of this invention has been described in connection with magnesium alloys containing from 3 to 10 percent of aluminum, such as alloys bearing the ASTM designations AZ31, A261, and AZ80, for example. These alloys nominally contain, besides the magnesium base metal, 3 percent Al, 1 percent Zn; 6 percent Al, 1 percent Zn; and 8 percent Al and about /fi percent Zn, respectively.

However, the method of this invention is applicable to the heating of billets of other magnesium alloys, bearing in mind that the hot-short temperature range changes Patented Apr. 2, 1957 3 with the, constituentsxof; the alloy and that the heating ratesmay have to be altered somewhat.

The invention is applicable to billets of various shape as well as to cylindrical billets previously mentioned in connection with the invention.

That which is claimed is:

1. The method of'heating billets of magnesium alloys having av thickness greater than 16 inches, comprising elevating the surface temperature of the billet at a rate approximating 100 F. per minute until the surface temperatureis near the hot-short temperature for the aiioy,

and then elevating the surface temperature at a rate of 5 to per minute at least until the center-to-surface temperature difference is less than 120 F.

2. The method of inductively heating billets of magnesium-alurninum-zinc alloys which are at least 16 i shes in diameter, comprising elevating the surface temperature of the billet at a rate approximating 100 F. per minute until the surface, temperature is near to the hot-short temperature of the billet and then elevating the temperature of the surface at a rate of between 5 and 10 F. per minute at least until the center-to-surface temperature difference is not greater than 120 F.

3. The method of heating billets of magnesiumaluminum-zinc alloys having a thickness greater than 16 c mp ising. ,eley th etbe. surtase. emperature. at

the billet at a rate approximating F. per minute nesium-aluminum-zinc alloys which are-at least 16 inches,

in thickness, comprising elevating thesurface temperature of the billet at a rate approximating 100 F. per minute until the surface temperature of the billet is about 650 F, and then elevating the temperature of the surface at a rate of between 5 to 10 F. per minute until the center-to-surface temperature difference is not greater than F.

References Cited in the file of this patent UNITED, STATES PATENTS Strickland May 14, 1946 Dreyfus Apr. 20, 1954 

1. THE METHOD OF HEATING BILLETS OF MAGNESIUM ALLOYS HAVING A THICKNESS GREATER THAN 16 INCHES, COMPRISING ELEVATING THE SURFACE TEMPERATURE OF THE BILLET AT A RATE APPROXIMATING 100*F. PER MINUTE UNTIL THE SURFACE TEMPERATURE IS NEAR THE HOT-SHORT TEMPERATURE FOR THE ALLOY, AND THEN ELEVATING THE SURFACE TEMPERATURE AT A RATE OF 5* TO 10* PER MINUTE AT LEAST UNTIL THE CENTER-TO-SURFACE TEMPERATURE DIFFERENCE IS LESS THAN 120*F. 